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Impedimetric immunosensor based on humidity sensing properties of BST composite film

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Author

Mahdi Rasouli

Date of Issue

2008

School

School of Electrical and Electronic Engineering

Abstract

In this project, a label-free impedimetric immunosensor with a signal amplification mechanism based on humidity sensing properties of Barium Strontium Titanate (BST) composite film has been developed.
The immunosensor was constructed as a capacitor structure. A layer of BST composite film was deposited onto a platinum coated silicon substrate, and then were modified by Poly(TMS-r-NHSMA) [TMS: 3-(trimethoxysilyl)propyl methacrylate; NHSMA: N-hydroxysuccinimide methacrylate] as a functionalized polymer to be able to bind to bio-molecules covalently. To demonstrate the sensing capability of this structure, Bovine Serum Albumin (BSA) protein was immobilized on the surface to detect its conjugate protein, anti-BSA, as a target analyte. The sample was then exposed to Phosphate Buffer Solution (PBS) buffer solution containing anti-BSA protein so that anti-BSA could bind with the immobilized BSA to form the antigen-antibody complex. The electrochemical detection was finally achieved by impedance spectroscopy after applying a drop of DI water between a top gold electrode and the sample as the bottom electrode.
It was observed that the formation of biorecognition complex could change the impedance of the system. Higher concentrations of anti-BSA could result in higher impedance values. These impedance increments (The sensing capability) were attributed to three parallel mechanisms upon the biorecognition reaction. First, the interfacial impedance of the system increases. Formation of the antigen-antibody complex reduces the charges transfer at the solid/liquid interface, and hence, results in an increase in the electron transfer resistance of the system. Moreover, the thickness of the interfacial layer also increases which causes a decrease in capacitance of the double layer and hence an increase in the interfacial impedance of the system. Second, impedance of the DI water increases. BST is a highly polar material which can dissociate water molecules into H+ and OH- ions. Immobilized biomolecules could prevent the water molecules from contacting BST grains and induce less water molecules to dissociate into mobile ions. This will result in a decrease in amount of charge carriers, and hence, an increase in the impedance of the DI water. Third, impedance of the BST composite film also increases when DI water is prevented from penetrating into the BST structure. This is due to the fact that the BST film is a humidity sensor and shows higher impedance for lower water contents.
The proposed immunosensor seems to be a good alternative to the conventional electrochemical immunosensors by utilizing two signal amplification mechanisms without using the harmful redox labels. Moreover, the proposed structure might be useful for detection of different analytes since the applied functionalized polymer is capable of immobilizing different proteins.